It is desirable to filter the patient's exhaled air to keep the intricate and difficult-to-clean bellows isolated from an individual's saliva and bacteria.
Bacterial filtration in pulmonary function testing presents multiple conditions which are difficult to satisfy simultaneously. For convenience and economic reasons, it is desirable that the filter be inexpensive enough to be disposable after use with a patient. It is also desirable that the volume of the filter be sufficiently small that it does not significantly affect the flow dynamics of the exhaled air within the testing device. Finally, it is important that any filtration device not introduce a flow restriction which might affect the results of the testing.
Small, disposable filters with filter media suitable for filtering bacteria from air are known. One example of such a commercial filter is a BB-50 filter manufactured by the present assignee and employing a fiberglass filter medium. The basic filter construction is similar to that disclosed in U.S. Pat. No. 3,815,754. Such a filter generates a sufficiently large pressure drop, however, that it is unsuitable for use in pulmonary function testing where it is important not to introduce significant flow restrictions. More open filter media, which might solve the pressure drop problem in a small and inexpensive filter, may not provide sufficient filtration, especially of the small aerosolized saliva droplets associated with high velocities encountered in deep, forceful breathing. A need exists for a small, inexpensive filter which provides sufficient filtration with an acceptably low pressure drop. A pressure drop of about 0.3 inches of water at a flow of about 500 liters per minute is considered to be acceptably low.
It is, accordingly, the primary object of the present invention to provide an inexpensive filter suitable for use in filtering human breath which has both a high bacterial retention efficiency and a low pressure drop.
It is a further object of the present invention to provide an effective, yet small and disposable filter unit suitable for use in pulmonary function testing.
These and other objects will be apparent from the following detailed description and associated drawings.